The invention relates to water treatment and, more particularly, to methods and apparatus for removing grit and grease from water.
City sewage, industrial waste water, and other types of water frequently include relatively large concentrations of both grit and grease. The treatment of waste water to remove impurities is generally performed with the aid of treatment devices whose operation and/or reliability can be adversely affected by grit and grease. Grit can be highly abrasive on pumps and the like, can clog filters and conduits, and can build up in tanks and thereby reduce the effective tank volume. Furthermore, excess grit can make handling and dewatering of sludge more difficult. Grease can form deposits on machinery to the point of interfering with proper operation of the machinery. Grease can also encourage the growth of microbes. Thus, it is common practice to subject waste water to an initial pretreatment operation to remove grit and grease prior to further treatment of the water to remove other impurities.
Grit is often removed by a process of sedimentation wherein water to be treated is introduced into a settling tank or the like and is allowed to remain long enough to permit grit, which is generally of greater specific gravity than water, to settle to the bottom of the tank. While in the settling tank, the water may be subjected to processes such as aeration designed to promote more-rapid settling of the grit, and additionally may be circulated or perturbed in such a way as to encourage grit to settle preferentially in certain regions of the tank. The grit is then removed from the bottom of the tank by any of various types of devices including scrapers, conveyors, pumps, and the like. One type of grit removal device that has been used in elongate grit settling channels is a traveling bridge-type device that travels lengthwise back-and-forth along the grit settling channel. The traveling bridge-type device supports a device that extends to the bottom of the channel and removes grit upwardly through a conduit and discharges the grit outside the grit channel.
A combined grit-and-grease removal system that has been developed by the assignee of the present application employs a grit channel and traveling bridge-type grit removal device as described above, in combination with a grease channel that receives degritted water from the grit channel and processes the water to remove grease. More specifically, grease is removed from the surface of the water in the grease channel by a skimmer assembly that is connected with the traveling bridge-type grit removal device so as to travel back-and-forth along the length of the grease channel, which extends parallel to the grit channel. The skimmer assembly collects grease that is floating on the surface, and a grease hoist assembly removes grease from the skimmer and carries it upwardly and out of the grease channel, where it is deposited into a grease bucket. The grit-and-grease removal system having the traveling skimmer assembly can be used only in installations in which the grease channel affords an unimpeded path for the passage of the skimmer. Furthermore, the traveling skimmer works only intermittently when the traveling bridge makes an excursion along the channels.
Accordingly, it would be desirable to provide a grit-and-grease removal system having improved grease removal components that would not limit the application of the system to installations providing an unimpeded path along the length of a grease channel. It would also be desirable to provide a grit-and-grease removal system capable of providing continuous grease removal.
The above needs are met and other advantages are achieved by the present invention, which provides a system for removing grit and grease from water in which grease removal is effected without the use of any skimmer assembly that travels the length of the grease channel, and in which grease removal can be carried out continuously.
In accordance with a preferred embodiment of the invention, a grit-and-grease removal system includes an elongate grit channel configured to receive water from a source, a traveling grit removal device operable to traverse a length of the grit channel and remove grit settled on the bottom of the grit channel, an elongate grease channel arranged to receive degritted water from the grit channel, and a plurality of grease skimmers positioned in the grease channel so as to be above an upper surface of water contained in the grease channel. The grease skimmers are spaced apart along a length of the grease channel and are adapted to receive fluid from a source, each grease skimmer having fluid discharge orifices oriented to direct fluid obliquely onto the surface of the water in a direction toward a discharge end of the grease channel so as to move grease floating on the surface toward the discharge end. The system also includes a grease removal device located at the discharge end of the grease channel and operable to remove grease from the water surface and deposit the removed grease outside the grease channel.
The system is thus amenable to installations in which it would be difficult to employ a traveling grease skimmer because of structure impeding the free passage of a skimmer. Additionally, in the prior grit-and-grease removal system employing the traveling grease skimmer, it was generally necessary to place the grease channel immediately adjacent and parallel to the grit channel and to have the length of the grease channel coextensive with that of the grit channel so that when the traveling grit removal device was traversed along the grit channel, the grease skimmer was also traversed along the grease channel. In the present invention employing non-traveling grease skimmers for grease removal, it is no longer necessary to make the grease channel coextensive with the grit channel, although it may still be advantageous to do so for other reasons. The system of the present invention also enables continuous grease removal, and does so at a relatively low capital cost inasmuch as grease skimmer devices can be made relatively inexpensively.
In a further preferred embodiment of the invention, the grit channel also includes aeration devices disposed in the grit channel so as to be submerged in water contained therein. The aeration devices are adapted to receive air from an air supply and to release air into the waste water. A first set of aeration devices impart motion to the grit/water mixture in the xe2x80x9cdead zonexe2x80x9d adjacent the front wall of the grit channel where the water to be treated flows into the channel, so as to prevent grit from immediately settling out and accumulating against the front wall. Farther down the grit channel, additional aeration devices promote settling of grit to the bottom of the channel. Preferably, the fluid used by the grease skimmers comprises air, and the grease skimmers and the aeration devices are supplied with air from a single common air supply. Thus, in installations employing aeration for grit removal, it is relatively simple and inexpensive to incorporate grease removal in accordance with the present invention since air for the grease skimmers is already available from the air supply used for the aeration devices.
Alternatively, the grease skimmers can employ fluids other than air, including other gases, or liquids such as water. For example, water can be pumped from one of the channels (preferably from the grease channel) and supplied to the grease skimmers.
The grease skimmers advantageously can be formed by a plurality of pipes extending transversely across the grease channel and spaced apart along its length. Each pipe preferably includes a plurality of spaced-apart orifices or nozzles formed through its side wall for discharging fluid onto the surface of the water. By appropriate location of the orifices, fluid can be impinged onto the water surface across substantially the entire width of the grease channel.
The grease removal device for removing grease from the water surface at the discharge end of the grease channel preferably comprises an inclined conveyor that lifts grease upward and away from the grease channel and allows any water also removed by the conveyor to continually flow back down the conveyor into the grease channel. Thus, the conveyor preferably performs a dewatering operation on the removed grease. Suitable conveyors include continuous belt conveyors and screw-type conveyors. The grease skimmers in concert with the conveyor enable grease to be removed on a continuous basis, independent of the operation of the traveling grit removal device.